Over the last few decades, with the resources on the earth being depleted daily, investment in exploring alternative energy sources has increased significantly. One of these alternative energy sources is solar energy which has drawn much attention from the energy industry. Photovoltaic cells (PV), which convert sunlight directly into electricity, were first exclusively used in space, as far back as the late 1950's, to power satellites' electrical systems. Since then, these PV cells have been used in a wide variety of fields, from calculators to emergency road signs, call boxes and even buoys. The technology continues to be used in new devices all the time, from sunglasses to electrical vehicle charging stations. These devices never need batteries and as long as there is enough light, they seem to work forever.
PV cells are made of semiconductors such as silicon, which is currently used most commonly. Basically, when sunlight strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely.
PV cells also all have one or more electric field that acts to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, it is possible to draw that current off for external use, for example, to power a calculator. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power (or wattage) that the solar cell can produce.
The field forms when the N-type and P-type silicon come into contact. Suddenly, the free electrons on the N side start to fill the openings on the P side. Eventually, equilibrium is reached, and there is an electric field separating the two sides.
When light, in the form of photons hits the solar cell, its energy breaks apart electron-hole pairs. Each photon with enough energy will normally free exactly one electron, resulting in a free hole as well. If this happens close enough to the electric field, the field will send the electron to the N side and the hole to the P side. This causes further disruption of electrical neutrality, and if an external current path is provided, electrons will flow through the path to the P side to unite with holes that the electric field sent there. The electron flow provides the current, and the cell's electric field causes a voltage. With both current and voltage, there is power, which is the product of the two.
Since silicon is a very shiny material, which can reflect photons rather than absorbing them, an antireflective coating is applied to reduce those losses. The final step is to install something that will protect the cell from the environment—often a glass cover plate. Solar panels are generally made by connecting several individual cells together to achieve useful levels of voltage and current, and putting them in a sturdy frame complete with positive and negative terminals.
To maximize reception of solar energy, the solar panel is normally in an elevated position and angled. This, combined with exposure to the elements, creates a need for frequent cleaning, especially since the efficiency of electricity generation is reduced due to the amount of dust and dirt accumulated on the transparent covers of solar panels. Another problem that is encountered is that, if the day is cloudy and sunlight does not hit the solar panel, then the efficiency of the electricity generation is also reduced. Or alternatively it may be a combination of the two factors. Thus, it is difficult to determine which of the factors is responsible for the reduced efficiency in the electricity generation.
In order to solve the first problem (of determining if the solar panel requires cleaning) the idea of comparing two sensors in order to compare a duty measurement to a reference measurement is well known in the art, for example, in US2009266353. This document discloses a method for automatically cleaning a solar panel utilizing an automatic cleaning system. An environmental intensity of sunlight in the outside environment is obtained with an environmental light sensor, and a transmitted intensity of incident sunlight throughout the protection panel is obtained using a transmission light sensor. The difference value between the environmental intensity and the transmitted intensity is then detected and by comparing the detection difference value with a predetermined value, it can be determined if the solar panel requires cleaning or not. If cleaning is required, various methods can be employed, including manual cleaning or by the use of an automatic cleaning device.
The usual problem in the prior art is that ambient sensors are prone to dust accumulation by themselves and so the indication of dirt on the solar panels is no longer reliable.